It is your quilt and your label...you aren't judged on it, nor is it integral for showing purposes (like a hanging sleeve). Put it wherever you want. The purpose of the label is to identify the maker and establish the history of the quilt...the more info you can put on the label, the better. There is no hard/fast rule for placement...you might not want to cover some intricate quilting that exists in the lower corner, so you could put it behind an applique element in the middle of the quilt back. It doesn't matter and you won't be judged poorly due to placement. Additionally, if you hate the process of hand-stitching a label (hard to turn a bed-size quilt to get a grip on the inner 2 sides of the label), then use liquid-stitch to attach it. Quick, easy, and permanent.